Last year, I attempted the creation of my own Halloween costume, and while doing so, drew up various other options, that I thought might be fun to put up online. As costume design doesn't really fit within the stuff I'd put on my other blogs, I thought I'd collect it here.
In general I just sketch bits over mannequin line art, there's a few base images out online for non-commercial use, and I find them handy to use to visualise how the costume can look. This can lead to the costumes looking a bit transparent, but you just have to ignore the model outline under the clothing.
The first base used here is by 'faith-npp32' from deviant art, here
I also like the bases from Sam Hope, here